Based on the number of views to date there's been a lot of passing interest in this item.
I might be wrong, but there doesn't seem to be any significant relationship between this item and any other known about Air Pistol designs. If that's right it makes it a "one off rather than a prototype". At a glance it looks as if most of this item could be made up from salvaged parts using rudimentary tools except from the frame and the cocking link which at a guess could well have taken ages to make (unless again, they too are parts salvaged from something else).
Even with a reasonable workshop and based on just looking at it, it would take days to make, and yet it's worth very little today. I can't help thinking that it was a labour of love made for a young lad in austere times.
JMHO, Vic Thompson.